In diseases that involve mucus secretion and movement in the small airways, such as chronic bronchitis, cystic fibrosis or asthma, liquid plugs form occluding bridges that obstruct the airway and disrupt gas exchange. In response to cough, these bridges move and the airway is reopened, with the transmission of mechanical forces to airway epithelial cells. Similarly, in the setting that involve both the airway and alveolar space, such as pneumonia or congestive heart failure, or mechanical ventilation with low tidal volumes, there is cyclic closure and reopening of smaller airways, which may be recognized as crackle sounds heard easily with a stethoscope. The cellular-level effect of the explosive transient pressure waves created by these reopening events, however, has not previously been investigated despite the likelihood that the associated plug rupture produces large stresses and is a major cause of lung injury. This proposal will investigate, experimentally and theoretically, the detrimental effect of fluid mechanical stresses on airway epithelial cells during airway reopening using a micro-engineered airway. The specific hypothesis is that the movement and rupture of liquid plugs in the small airway system during airway reopening will generate large fluid mechanical stresses and damage airway epithelial cells, and that even normally sub-lethal amounts of fluid mechanical stress will become lethal in the presence of other insults such as bacteria or hyperoxia-mediated inflammation, expanding the region and severity of injury. The specific aims of this proposal are: 1. Design and fabrication of a biomimetic microfluidic system to perform in vitro culture of airway epithelial cells under physiological air-liquid interface conditions. 2. Generation of liquid plugs with physiological propagation velocities and rupture frequencies within the engineered microfluidic small airways, and combined computational and experimental assessment of the resulting fluid mechanical stresses and their effect on cell injury. 3. Investigate synergistic cellular damage caused by combination of liquid plug propagation/rupture- mediated fluid mechanical stresses and bacterial infection or hyperoxia-mediated inflammation. Also, evaluate the effect of surfactant as a countermeasure to reduce cellular injuries.